The scenario can be better understood if we consider the setting where we simply remove the pre-order traversal CNN (Line 4, Table REF ). Both based on a tree-based CNN layer, the LSTM component yields a 4% lower accuracy than simply without LSTM.
This implies that RNNs are not suitable to this task, which is probably because a program contains too many tokens and AST nodes. An RNN applied to such a long sequence can be very difficult to train [1]}, achieving significantly worse performance.
